Flow intermittence controls leaf litter breakdown in a French temporary alluvial river: the “drying memory”

Abstract

The inputs and breakdown of terrestrial leaf litter in streams is a fundamental ecological process that sustains in-stream foodwebs and secondary production. In temporary rivers, litter breakdown is reduced during dry phases, but the long-term effect of alternating drying and wetting cycles on litter breakdown is still poorly understood. We tested the hypothesis that leaf litter breakdown (LLB) in temporary rivers is primarily controlled by flow permanence (the number of flowing days over a given period expressed in %), and that drying events affect LLB during leaf fall periods through reduction of microbial activity and the modification of aquatic invertebrate assemblages. LLB rates (k), microbial activity and invertebrate assemblages were determined in winter at ten crosssections scattered along a flow permanence gradient on the temporary Albarine River, France. Results demonstrated that summer drying events affected the breakdown process for up to 6 months after flow has resumed in the river. LLB rates decreased exponentially with decreasing flow permanence, and with increasing drying event duration and frequency. These exponential relationships were observed for flow permanence variables calculated for the river for both 24-years and 1-year time periods prior to the experiment. A decrease in flow permanence from 100 to 85% led to a four-fold decrease in leaf litter breakdown rate. Microbial activity, which typically did not differ between cross-sections, failed to explain the betweencross-section differences in k. By contrast, invertebrate assemblages and, shredders, in particular, decreased exponentially with decreasing flow permanence and with increasing drying event duration and frequency.

Recent perspectives on temporary river ecology

“…The fact that leaf litter breakdown was detectable under permanently dry conditions could be explained by alternative mechanisms such as leaching and photodegradation (Dieter et al 2011). Datry et al (2011) found that that dry phase duration influenced litter decomposition rates primarily because of the negative effects of the dry phase on invertebrate shredders, and that this effect extended long after rewetting, as if there was a dry event ''memory''. They observed that drying events affected the performance of the decomposing biota and thus the decomposition rates of leaf litter.…”

Natural variation in immersion and emersion affects breakdown and invertebrate colonization of leaf litter in a temporary river

“…This knowledge gap limits our understanding of the range of pathways that exist for allochthonous organic matter processing in rivers. There is evidence suggesting that emersion inhibits colonization of leaf litter by aquatic leafshredding invertebrates, and that immersion duration and frequency influence litter breakdown in temporary river habitats (Boulton 1991;Langhans and Tockner 2006;Datry et al 2011). The effects of emersion on micro-organisms colonizing leaf litter and on microbial decomposition are less clear.…”

“…The repeated onset and cessation of flow causes temporary rivers to contract and expand in the lateral and longitudinal dimensions, alternately immersing and exposing channel sediments. Temporary rivers are very common, but the state of knowledge concerning organic matter dynamic in these rivers is poor (Larned et al 2010a;Datry et al 2011). This knowledge gap limits our understanding of the range of pathways that exist for allochthonous organic matter processing in rivers.…”

“…Results of some studies indicate that alternating emersion and immersion stimulates micro-organism colonization and activity (Swift et al 1979;Ryder and Horwitz 1995). Results of other studies suggest that the primary effect of emersion is to reduce microbial activity (Boulton 1991;Datry et al 2011). Microbial decomposition clearly occurs during emersed periods, particularly when the litter is partially rehydrated by high humidity, rainfall or dew (Kuehn et al 2004;Langhans and Tockner 2006).…”

Microbial communities in an ephemeral stream system and the implications of saline mine discharge

“…Flow is highly variable within these streams, and has been shown to affect organic matter breakdown (Fischer et al 2002, Claret and Boulton 2003, Datry et al 2011. Organic matter in this study was based on observed differences in levels of detritus (as leaf litter) and coal, however types of organic matter and carbon have not been quantified.…”